1. Field of the Invention
The present invention relates to drive control of an image forming apparatus which forms an image on a recording medium.
2. Description of the Related Art
Color misregistration is one of the criteria for determining output image quality of a color image forming apparatus in which high-quality image output is demanded. To reduce such color misregistration, the image forming apparatus may form toner patches of each color on an intermediate transfer belt and detect color misregistration using a registration detection sensor to detect the position of the toner patches. The color image forming apparatus then changes timing of forming each color image on a photosensitive drum based on the detection result.
Further, velocity fluctuation of the intermediate transfer belt causes color misregistration in an image forming apparatus which sequentially activates image forming units including the photosensitive drum. If velocity fluctuation is generated in a transfer conveyance belt or the intermediate transfer belt, power applied on the belt from an image bearing member becomes different at respective transfer nips of the image forming units for each color. As a result, a pulling force or a pressing force is applied on the belt between the transfer nips of the image forming units of each color, which causes a difference in the velocities of the belt passing through each of the transfer nips. Color misregistration is thus generated. When peripheral velocities of the photosensitive drum and the intermediate transfer belt are different, a friction coefficient between the photosensitive drum and the intermediate drum changes according to the presence or absence of toner in a primary transfer nip portion. Such change in the friction coefficient also causes a change in a tangential force, thus leading to generation of color misregistration.
To solve such a problem, there is a technique for preventing velocity fluctuation of the intermediate transfer belt from affecting the image. More specifically, load fluctuation is generated when charging, developing, and transferring processes are switched on and off in the image forming unit. In such a technique, the processes are switched on and off when a visualized image is not being transferred from the photosensitive drum to the intermediate transfer member.
However, in the above-described method, time for performing the charging and developing processes becomes longer, so that the lifetime of the image forming unit becomes immoderately shortened.
Further, it has been determined by inventors that a relation between the peripheral velocity difference of the photosensitive drum and the intermediate transfer belt, and the color misregistration caused by the velocity fluctuation of the intermediate transfer belt changes due to other factors. An example of such factors is usage of the photosensitive drum and the intermediate transfer belt. It is thus necessary to consider the factors which affect the degree of change in the tangential force to reduce color misregistration.